The use of cutting tools having replaceable cutting inserts began as long ago as 1917, when Fred P. Lovejoy invented the use of replaceable blades in order to obtain the economic advantages of having to replace only the dull portion of the tool, not the entire tool itself.
The next major improvement in this process was the invention of the tungsten titanium carbide insert by Philip M. McKenna in 1938, especially for use in milling machines. A typical milling machine is an apparatus that features a rotating mill head having a number of indexable cutting inserts, where the rotating head is passed over the workpiece to remove material from the workpiece.
Since the time of the invention of the carbide cutting insert, tremendous effort has been made to understand the myriad factors affecting the performance of cutting inserts. These factors include insert geometry, insert construction, temperature, cutting forces, workpiece material characteristics, and chip control. In fact, Kennametal, Inc., founded by inventor Philip McKenna, lists thousands of insert geometry/size/composition/coating combinations in order to meet the requirements of differing applications. Despite the overwhelming number of inserts that are available, standard inserts are still "standardized".
Inserts having various basic geometric shapes can be selected, with the most common being the square, triangle, rhomboid, and hexagon. Indexable inserts have a plurality of cutting edges that increase the working life of the insert. Once a particular cutting edge has become dull, the insert is indexed in its holder to expose a new cutting edge. Once all cutting edges have dulled, the insert is believed to be useless and is thrown away.
Sophisticated technology has been developed concerning the selection of materials and geometries of cutting inserts resulting in vast improvements in durability and cutting efficiency, with attendant cost increases. Despite the advances in insert technology and increased costs of cutting inserts, these inserts are still defined and treated as "throw-away", even in the USA Standard Indexable Inserts for Cutting Tools (894.25-1969).
Indexable inserts generally are manufactured by pressing powdered metal into various shapes and then sintering in a furnace to establish the hard quality of the material. In some applications, the need for inserts having a high dimensional accuracy has led to attempts to grind the edges of new inserts.
U.S. Pat. No. 4,463,524, issued to Schott et al. on Aug. 7, 1984, discloses a dedicated machine for finish grinding indexable tool inserts that includes a pivoting grinding wheel that rotates in a horizontal plane and a tool insert holder that rotates the insert during grinding such that all sides and edges of the insert are ground. However, the machine disclosed in the Schott et al. patent is not adapted for the remanufacture of tool inserts as all sides of the insert are ground which results in a ground new insert of the correct size. When applied to the remanufacture of a previously used insert, however, the result is an insert having a smaller overall size than that of the original. Since a remanufactured insert reground by the disclosed machine is smaller than original, the inscribed circle (defined as the largest internal circle that can be drawn such that all sides of the insert are tangent to that circle) is also reduced and the remanufactured insert does not fit accurately within the tool holder. This drawback can be countered by the use of shims, complicating the installation process. In addition to its inapplicability to the remanufacturing of dulled inserts, the machine of Schott et al. is a dedicated machine which may only be used to grind inserts and thus would not be a cost effective solution for most machine shops and other small scale users of tool inserts.
Similarly, U.S. Pat. No. 5,421,679, issued to Pantzar et al. on Jun. 6, 1995, discloses a grinding operation that is intended to be used only along an area against a locating surface of a machine tool. Like those of Schott et al., the inserts disclosed by Pantzar et al. require a very high degree of dimensional accuracy that is met by after-grinding the surface(s) adjacent to the cutting edge after the insert is sintered. However, it is taught that this grinding (called contour grinding) causes adverse modifications in the microgeometry of the insert. Thus, Pantzar et al. discloses what has been a commonly held truism in the art for more than 45 years: inserts cannot be sharpened once dull without damaging the geometry of the insert. This perceived damage is one reason for the mistaken belief that inserts must be thrown away once dull.
The belief that dull inserts must be thrown away has been reinforced by failed efforts to regrind used inserts to gain additional useful life before the insert must be discarded. The most ambitious of the regrinding attempts to reclaim dulled inserts, frequently referred to as "downsizing", is provided by North American Carbide, Inc. of Broken Arrow, Okla. In this process, the "dulled"/used insert is reground using specially adapted grinding machinery so that an insert that is virtually identical to the original, only slightly smaller, is obtained. Unfortunately, the success of this process has been rather limited as the reduction in overall size of the insert results in the same problems as would be encountered by using the machine of Schott et al., i.e. reduction of the size of the inscribed circle resulting in difficulty in accurately clamping the insert within the tool holder. Many companies initially embraced this concept, recognizing the significant financial advantage of being able to increase the life span of an insert. However, the difficulties encountered with clamping the inserts to fit accurately within the tool holder once it had been refurbished quickly diminished the enthusiasm for the approach. Consequently, the belief that cutting inserts are only capable of a one-time use and then must be thrown away continues to prevail after all these years.
An apparatus and process for grinding a dulled tool insert that may be used with a standard grinding machine, that will provide sharpened edges having cutting performance equal to, or exceeding, a new insert, and that will provide an insert which can be held in a tool holder without the use of shims or special holders is not known in the prior art.